1. Field of the Invention
The present invention relates to a novel organic compound and its polymers. It further relates to a light emitting device which has a light emitting layer composed of a light emitting material and transfers the energy of applied voltage directly to light energy.
2. Description of the Related Art
Electroluminescence (EL) of organic materials was observed in anthracene single crystals by Pope et al. in 1963 (J. Chem. Phys. 38 2042). With the use of a solution electrode system having a high injection efficiency, comparatively strong injection type electroluminescence was successfully observed by Helfinch and Schneider in 1965 (Phys. Rev. Lett. 14 229).
Since then studies have been made on organic light emitting substances each composed of a conjugated organic host emitting material and a conjugated organic activator having a condensed benzene ring, as reported in, for example, U.S. Pat. No. 3,172,862, No. 3,173,050, J. Chem. Phys. 44(1966)2902, J. Chem. Phys. 50(1969)14364, J. Chem. Phys. 58(1973)1542 or Chem. Phys. Lett. 36(1975)345. Each of these organic light emitting substances is, however, a single layer having a thickness of more than 1 .mu.m and requires a high electric field for light emission. Accordingly, attempts have been made to develop thin film devices obtained by vacuum deposition (e.g., Thin solid Films 94(1982)171, Polymer 24(1983)748, J. Appl. Phys. 25(1986)L773). While such thin film devices were effective for reducing the drive voltage, they had not yet achieved a sufficiently high luminance for practical use.
Recently, Tangs et al. (Appl. Phys. Lett. 51(1987)913; and U.S. Pat. No. 4,356,429) invented an electroluminescent device obtained by laminating two extremely thin layers (a charge transport layer and a light emitting layer) between an anode and a cathode so as to achieve a high luminance with a low drive voltage. Such laminated organic electroluminescent devices have been thoroughly studied thereafter as reported, for instance, in Japanese Patent Laid-Open No. 59-194393, U.S. Pat. No. 4,539,507, U.S. Pat. No. 4,720,432, Japanese Patent Laid-Open No. 63-264692, Appl. Phys. Lett. 55(1989)1467 and Japanese Patent Laid-Open No. 3-163188.
Laminated organic electroluminescent devices are usually obtained by vacuum deposition, whereas devices having a relatively high luminance have been reported to be obtained by casting (Proceedings of the 50th Academic Lecture of The Japan Society of Applied Physics, 1006(1989); and Proceedings of the 51st Academic Lecture of The Japan Society of Applied Physics, 1041(1990)).
It has been also reported that electroluminescent devices having a mixed one-layer structure are obtained by dip coating with a solution containing a mixture of polyvinyl carbazole as a hole transport compound, an oxadiazole derivative as an electron transport compound and coumarin 6 as an illuminant. These devices provide a relatively high luminous efficiency (Proceedings, of the 38th Applied Physics-related Association Lecture, 1086(1991)).
As described above, great advantages have been made recently in organic electroluminescent devices, indicating that they can be applied to a broad spectrum of applications.
The studies on these electroluminescent devices, however, have been conducted too recently to give sufficient results on selection of materials or production methods of devices. Light emission with a further high luminance or having a wide variety of emission wavelengths have not yet been sufficiently achieved. In addition, these electroluminescent devices are still insufficient in light emission properties, and especially luminous efficiency. Therefore, there is a desire to further improve and investigate such devices.